2.1. BILATERAL VOCAL CORD PARALYSIS
Bilateral vocal cord paralysis ("BVCP") is a condition in which both recurrent laryngeal nerves ("RLN") are severed and all the intrinsic laryngeal muscles, except for the cricothyroid, are paralyzed (Holinger et al., 1976, Ann. Otol. Rhinol. Laryngol. 85: 428). The vocal cords assume a position near the midline which allows for an acceptable voice. Because of the loss of function of the posterior cricoarytenoid muscle ("PCA"), however, the vocal cords no longer abduct during inspiration. If left untreated, the individual ultimately suffocates. The standard initial therapy for this condition is to bypass the obstruction by performing a tracheostomy. Although crudely efficient in restoring an airway, tracheostomy requires a great deal of care and involves a number of serious social disadvantages and social complications (Myers and Stool, 1985, in Tracheotomy, Myers et al., eds, New York, Churchill Livingston).
Numerous surgical procedures have been proposed to rehabilitate the patient with BVCP and allow for removal of the tracheostomy tube. Currently, the most common clinical procedures include vocal cord lateralization (Kirchner, 1979, Laryngoscope 89: 1179), and arytenoidectomy (Woodman, 1946, Arch. Otolaryngol. Head Neck Surg. 43: 63), which widen the glottic opening by permanently altering the vocal cord. The disadvantage to these procedures is that the widened glottis increases the patient's susceptibility to aspiration while simultaneously decreasing the quality of the voice.
Another means of rehabilitation involves reanastomosis of the RLN. However, the RLN contains both abductor and adductor fibers and upon its reanastomosis, these fibers regenerate in an indiscriminate manner, so that the resulting laryngeal movement is uncoordinated and synkinetic (Doyle et al., 1967, Laryngoscope 77: 1245; Siribodhi et al., 1963, Laryngoscope 73: 148).
An entirely different approach to rehabilitation attempts to restore the function of the PCA muscle by reinnervation with nerves presumed to carry neural activity synchronous with inspiration. These nerves are either connected to the severed distal stump of the RLN or the RLN nerve branch which enervates the PCA, or are implanted directly into the PCA. Nerves used in these procedures include the phrenic, cervical nerve trunk, ansa cervicalis, and nerves to the cricothyroid, sternohyoid, and omohyoid muscles (Jacobs et al., 1990, Ann. Otol. Rhinol. Laryngol. 99: 167; Rice and Burstein, 1983, Arch. Otolaryngol. Head Neck Surg. 109: 480; Hengerer, 1973, Arch. Otolaryngol. Head Neck Surg. 97: 247; Tucker, 1983, Laryngoscope 93: 295).
It has been theorized that restoring denervated PCA function may be accomplished with a functional electrical stimulation system (Sanders, 1991, Otolaryngologic Clinics of North America 24: 1253-1274, citing Broniatowski et al., 1986, Otolaryngol. Head Neck Surg. 94: 41; Broniatowski et al., 1985, Laryngoscope 95: 1194; Hollinger et al., 1976, Ann. Otol. Rhinol. Laryngol. 85: 4285; Obert et al., 1984, Arch. Otolaryngol. 110: 88; Otto and Templer, "Coordinated pacing of vocal cord abductors in recurrent laryngeal nerve paralysis", presented at the Research Forum of the American Academy of Otolaryngology, Las Vegas, September, 1984; Sanders and Biller, "Considerations in the development of a laryngeal pacer", presented at the Research Forum of the American Academy of Otolaryngology, Atlanta, September, 1985; Zealar and Dedo, 1977, Acta Otolaryngol. 83: 514; Zrunek et al., "Direct electrical stimulation of the posterior cricoarytenoid muscle" in Proceedings of the 1st Vienna International Workshop on Functional Electrostimulation, Vienna, Austria, Oct. 19-22, 1983; Zrunek et al., "Functional electrical stimulation in bilateral recurrent nerve palsy in sheep: Functional and biochemical results" in Proceedings of the 2nd Vienna International Workshop on Functional Electrostimulation", Vienna, Austria, Sep. 14-21, 1986).
Of all the possible tissues that could be stimulated by such a system, denervated muscle is the least studied (Sanders, 1991, Otolaryngologic Clinics of North America 24: 1253-1274), and has been associated with a variety of technical problems.
Sanders et al., 1989, Ann. Otol. Rhinol. Laryngol. 98: 339-345 reports successful transmucosal electrical stimulation of laryngeal muscles in dogs, and states that transmucosal stimulation appears promising as a diagnostic technique for correlating particular vocal cord movements and thresholds of activation with specific laryngeal disorders. However, this technique cannot be used therapeutically because the stimulation of mucosa causes pain in the awake patient.
Studies of stimulation of denervated PCA in dogs suggest that in order to achieve a sufficient firing frequency, an overly large power supply would be required (Sanders, 1991, Otolaryngologic Clinics of North America 24: 1253-1274, see pp. 1257-1262). It has been suggested that this problem might be solved by limiting the duration or frequency of firing, or by augmenting artificial stimulation with physiologic stimulation recruited from intact nerves.
Another problem to be addressed is the undesirable spread of current to stimulate muscles other than the PCA (Id., pp. 1262-1263). In BVCP, the intrinsic muscles of the larynx are paralyzed and sensation is lost in the subglottic larynx and trachea, so that high current flows required to stimulate the denervated PCA can easily spread to excite these structures. It has been suggested that the current might be further localized by manipulating such factors as the size, location, and orientation of the electrodes, variables which constitute what has been referred to as the "electrode array". A pilot study of chronic stimulation of denervated PCA in dogs indicated that chronically stimulated muscle resisted atrophy and was more responsive to electrical stimulation than muscle that was not chronically stimulated (Id., pp. 1270-1272).
2.2. FUNCTIONAL ELECTRICAL STIMULATION
Functional electrical stimulation ("FES") is the application of stimulation devices to nerves to rehabilitate neurological deficits. The most successful FES system to date is the cardiac pacer which has become a routine part of cardiac disease therapy (Lynch, 1982, "Cardiovascular Implants" in Implants, Lynch, ed., New York, Van Nostrand Rheinhold). There are a variety of other FES systems, however. The most heavily researched are FES systems to restore locomotion to paraplegics and arm motion to quadriplegics (Peckham, 1991, IEEE Trans. Biomed. Eng. 28: 530). Other motor control devices restore bladder control to paraplegics and diaphragm function to high quadriplegics (Erlandson, 1978, Scand. J. Urol. Nephrol. 44(Suppl.): 31; Glenn, 1976, Ann. Surg. 183: 566). There are also FES devices designed to rehabilitate sensory deficits, such as the cochlear implant (Hambrecht, 1979, Ann. Otol. Rhinol. Laryngol. 88: 729).
Neural FES devices which may be used to stimulate spinal cord, phrenic nerve, cochlea, paralyzed limbs or sacral nerve are available. Sacral nerve FES systems are currently being investigated for use in the management of pelvic dysfunctions which have proved refractory to appropriate conventional treatments. Such dysfunctions include urinary incontinence due to detrusor instability or sphincter instability, voiding difficulties such as dysfunctional flow and urinary retention, urgency/frequency syndromes associated with urethral syndrome and prostatism, recurrent urinary tract infections and pelvic pain.